This invention relates to a wind tunnel model mount system and more particularly to a flexible structure mount system for effectively and accurately determining the effects of angle of attack and airstream velocity on the flutter of a model aircraft or airfoil. More particularly, the angle of attack of a wind tunnel model and the airstream velocity are varied until the model experiences sustained sinusoidal flutter.
Flutter is the self-excited oscillation of an elastic body such as an aircraft wing, fuselage, empennage etc. in an airstream and may result in catastrophic structural failure. Flutter of an airfoil generally comprises pitch motion, the oscillatory rotation of the airfoil about its lateral axis, and plunge motion, the oscillatory vertical translation of the airfoil in a direction perpendicular to its chordline. The aerodynamic forces that are responsible for flutter motion are caused principally by the oscillatory aeroelastic deformation of the structure. For a particular structure, flutter motion depends on the mass and stiffness properties of the structure, the geometric shape and angle of attack, and the velocity and density of the airstream.
Flutter is a complex phenomenon and analytical methods to predict flutter are not sufficiently accurate; therefore, most flutter research is conducted experimentally in wind tunnels. To conduct basic wind tunnel flutter studies, it is desirable to use models which are as simple as practical yet still represent the important flutter parameters. A model mount system which provides for only pitch and plunge degress of freedom meets this requirement for many research studies. This is because the flutter characteristics of a rigid wing with only pitch and plunge degrees of freedom are very similar to the flutter characteristics of flexible wings which have very complex degrees of freedom. Such a model mount system must be strong enough to carry large steady state and dynamics loads. Previously, such model mount systems employed bearings and linkages which made the systems complex, unwieldy, and expensive to fabricate. Moreover, the bearings or linkages supporting the model inherently introduce undesirable damping which can change unpredictably as the load on the model changes especially at high airstream velocities and angles of attack, thereby inaccurately simulating the flutter motion of the structure. Attempts to obtain accurate test data by subtracting out the damping error or by ignoring it have proved laborious and inaccurate.
Accordingly, it is an object of the present invention to provide an improved wind tunnel model mount system with new and novel apparatus for supporting the model in the airstream and designed to accurately simulate the flutter motion of airfoils and aircraft, thereby enabling the design of an airfoil or aircraft which will not suffer catastrophic structural failure.
A further object of the invention is to provide a model mount system which, in combination with airflow within the wind tunnel and the model airfoil or aircraft, accurately simulates flutter oscillations at varying angles of attack and airstream velocities.
Another object is to provide a means for adjusting the model angle of attack.
A further object is to provide a model mount system which can carry large, steady-state and dynamic loads and includes linear elastic constraints to simulate pitch and plunge oscillations at high airstream velocities and which is insensitive to forces other than aeroelastic deformation.
Still another object is to provide a model mount system which may be used and adapted to a variety of aircraft and airfoil models including supercritical airfoils.
Yet another object is to provide a model mount system with a small amount of damping of the flutter oscillations.
Still another object is to provide a model mount system which is simple, lightweight, durable and inexpensive to fabricate.
Another object is to provide a system which employs a rigid airfoil or aircraft model with flexible supports that extend away from the wind tunnel wall and into the main airstream.